Are you taking into consideration the difference between a fluid ounce and the ounce as a unit of weight or mass? One oz weight = 28.3495 grams, while one fluid ounce H2O weighs 29.57 grams. Fluid oz and the mass that is equal to 1/16 of a pound are not interchangeable.
Therefore, one fluid oz of water, alcohol, mercury, oil, gasoline or anything else of differing specific gravity, will have differing weights.